Ill] 



HYDEOMEDUSAE 



59 



A large number of the floelenterata, called the Hydromedusae,v 

 agree with Hydra in all essential points of structure ; 

 the most important point of difference is that in 

 them the buds do not become separated, but remain 

 permanently in connection with the parent, and thus compli- 

 cated colonies are built up (Fig. 23). Other differences of less 



Hydro- 

 medusae. 



FIG. 24. Part of a branch of Obelia sp. To the left a portion is shown 

 in section. After Parker and Haswell. 



. Ectoderm. 2. Endoderm. 3. Mouth. 4. Coelenteron. 



5. Coenosarc. 6. Perisarc. 7. Hydrotheca, prolonged at base 



of hydroid as a shelf. 8. Blastostyle, a mouthless hydroid bearing 



medusa-buds. 9. Medusa-bud. 10. Gonotheca, part of perisarc 



which protects the medusa-buds. 



